The invention relates to a synthetic magnesium silicate having a layered crystalline structure, to a process for its preparation and to its use.
Synthetic hectorites are trioctahedral sheet silicates of the smectite type of the formula EQU [Mg.sub.6-x -Li.sub.x Si.sub.8 O.sub.20 (OH).sub.4-y F.sub.y ].sup.x(-)x /.sub.z M.sup.z(+)
where 0&lt;x&lt;1.2 and 0&lt;y&lt;4 and M is a cation having a valency z (z is 1, 2 or 3). Because of their rheological properties, they are widely used in thickening systems (thixotropic agents). A number of processes are known for preparing synthetic hectorites.
DE-A-11 84 742 describes a process for the preparation of a synthetic mineral of the hectorite type having improved rheological properties, in which a solution containing magnesium ions and an alkaline sodium silicate solution are introduced into a third solution containing lithium ions and fluoride ions, and then the reaction mixture is heated with agitation until a sample of the product therefrom, after filtration, washing and drying at 110.degree. C., forms a gel in a 5% by weight dispersion in water. The product is worked up and heated at from 110 to 450.degree. C. in order to develop useful aqueous gelling properties.
DE-A-15 67 503 describes an analogous process, although the product is reacted in aqueous solution with an amine salt prior to the heat treatment at from 110 to 450.degree. C. Both processes give products which are able to form organic adducts.
Both processes, however, have the disadvantage that considerably long reaction times are necessary and also that precise control of the quality of the final product is very difficult.
British patent GB-B-1 432 770 describes a process for the preparation of an anhydrous magnesium silicate having a crystalline structure similar to hectorite, in which firstly a magnesium salt is reacted with a sodium carbonate solution to give magnesium carbonate and then a sodium silicate solution is added to precipitate silicon dioxide. The resulting suspensions are then boiled for 1 hour at atmospheric pressure and then treated hydrothermally at 207.degree. C. for from 3 to 6 hours.
These processes produce fluoride-free products whose optical density and whose Bingham yield values (2% strength dispersion in tap water containing 140 mg of CaCO.sub.3 /l) are measured.
Another process for the preparation of fluoride-free anhydrous magnesium silicates is described in
DE-C-23 56 865. This involves dissolving a water-soluble magnesium compound in an aqueous medium and adding an aqueous alkaline solution of one or more sodium compounds to the aqueous medium to precipitate a water-insoluble magnesium compound, and then precipitating silicon dioxide in the aqueous suspension containing the water-insoluble magnesium compound. In this process, the magnesium compound and the silicon dioxide are precipitated together.
PCT/WO 93/11069 describes a process for the preparation of a synthetic hectorite, which involves preparing, under basic conditions, a precursor in solid form which already has the Mg/Si ratios of the subsequent hectorite. This precursor is prepared in a narrow pH range between 7.5 and 9 or between 9.7 and 10. The precursor is then crystallized in a hydrothermal method to give the actual hectorite product. Exact pH control and adherence to constant values is said to produce a product having improved properties.
In U.S. Pat. No. 3,586,478, a synthetic sheet silicate of the hectorite type is obtained by preparing an aqueous mixture of a water-soluble magnesium salt, sodium silicate, sodium carbonate or sodium hydroxide solution and a solution containing Li and F ions, and treating this mixture hydrothermally at the boiling point and atmospheric pressure for from 10 to 20 hours in order that the product crystallizes out. Following washing and drying, a product in which the cations and anions are present in certain quantity ratios relative to one another is obtained.
DE-A-16 67 502 describes a process for the preparation of a F-free synthetic clay mineral, which involves adding to a first aqueous solution which contains magnesium ions and lithium ions, a second solution which contains sodium carbonate and sodium silicate, with formation of a white precipitate. The resulting mixture is then heated, and after a relatively long period under pressure, a product crystallizes out, which is washed and dried.
In the process of GB-A-1 054 111, a synthetic clay mineral is prepared by heating an aqueous solution containing lithium ions and magnesium ions to boiling point and slowly adding a sodium silicate solution thereto. An aqueous sodium carbonate solution is then added. Following hydrothermal treatment, the product is filtered, washed, dried and ground.
EP-A-0 088 372 describes a process for the preparation of a synthetic hectorite which involves heating an aqueous solution of a magnesium salt to from 110 to 373.degree. C., adding lithium ions and/or fluoride ions and then adding a source of sodium and silicon.
DE-A-27 18 576 describes a very specific process for the preparation of a material comprising a fluorine-containing trioctahedral clay of the smectite type, in which, inter alia, a silicon dioxide sol is added. The Bingham yield points of the resulting products are only useful for a few areas of application.
U.S. Pat. No. 4,049,780 likewise describes a complex process for the preparation of synthetic silicates which also produces products which have inadequate Bingham yield points for many practical applications.
Many of the abovementioned processes have the disadvantage that the space-time yields are low and that the product properties cannot be uniformly reproduced very easily. Many processes also appear to be very time-consuming and economically unfavorable. In some cases, the resulting products and their properties vary very greatly as regards their composition and quality. Moreover, it is often not possible to prepare products which have certain values as regards yield points in the corresponding electrolyte solutions or which have high fiber affinity.